1) Field of the Invention
The present invention generally relates to a wet-type image forming apparatus such as a copy machine, facsimile machine or a printer and, more particularly, to a wet-type image forming apparatus in which toner in a liquid carrier applied on a latent image carrier is cohered prior to a transferring process.
2) Description of the Related Art
A wet-type image forming apparatus is known which uses a developing liquid comprising a liquid carrier and toner dispersed in the liquid carrier. A toner image is formed on a latent image carrier such as a photosensitive drum by using the developing liquid. The toner image is then transferred onto a transfer material such as a recording sheet or an intermediate transfer belt. In this apparatus, a phenomenon, which is referred to as electrostatic migration, of the toner in the liquid carrier is utilized so as to adhere the toner onto the latent image carrier during a developing process, or move the toner from the latent image carrier to the transfer material during a transferring process.
In the above-mentioned conventional wet-type image forming apparatus, a thin layer of the liquid carrier having 30-200 .mu.m thickness is formed on the latent image carrier when the developing liquid is applied onto the latent image carrier. If the developed toner image is transferred onto the transfer material by pressing the transfer material onto the latent image carrier in the above-mentioned condition, the toner image developed on the latent image carrier is deformed. The deformation of the toner image includes the collapse of an image and swelling of a line. Thus, the toner image cannot be precisely transferred onto the transfer material. This is because an excessive amount of the liquid carrier exists on the latent image carrier. If the excessive amount of liquid carrier exists on the latent image carrier, the toner of the toner image may move in the liquid carrier.
Additionally, when the transferring process is performed when the excessive amount of the liquid carrier exists, a large amount of liquid carrier is transferred onto the transfer material. Thus, there is a problem in that consumption of the liquid carrier is increased, and extra heat and extra time is required for drying the liquid carrier on the transfer material.
In order to eliminate the above-mentioned problems, the excessive amount of the liquid carrier is removed from the latent image carrier. The removal of the excessive amount of the liquid carrier can be achieved by providing a squeeze roller or a corona discharging device at a predetermined distance apart from the latent image carrier. The squeeze roller is rotated in a reverse direction of the rotating direction of the latent image carrier to catch the excessive amount of the liquid carrier. The corona discharging device generates ions having the same polarization with the toner and irradiates the ions toward the latent image carrier. These methods for removing the excessive amount of the liquid carrier are known in the art.
However, if too much amount of the liquid carrier is removed, a part of the toner image transferred onto the transfer material may possibly be removed and the toner image may have a blank. This is because the above-mentioned electrostatic migration is not sufficiently achieved when a sufficient amount of liquid carrier does not exist.
Japanese Laid-Open Patent Application No. 4-503265 discloses a method in which toner in a toner image formed on the latent image carrier is condensed before it is transferred onto a transfer material by an electric field formed between a roller and the latent image carrier. The toner image is condensed by an effect of the electric field. In this method, a sufficient amount of the liquid carrier is applied onto the latent image carrier and, thus, the roller always contacts the liquid carrier on the latent image carrier. Accordingly, a shearing stress is generated in the liquid carrier near a position at which the roller is most closely adjacent to the latent image carrier on the downstream side of the latent image carrier. This is because a part of the liquid carrier on the latent image carrier is separately carried by the roller. When the shearing stress is generated, the toner image of the latent image carrier is disturbed, and a part of the toner adheres on the latent image carrier other than the area where the toner image should be formed. This phenomenon is referred to as scattering of toner. The scattering of toner is generated particularly in a position near the trailing edge of the toner image on the latent image carrier. When a voltage is applied to the roller, the scattering of the toner is particularly increased. When the scattering of the toner is generated in the liquid carrier, sharpness at the edge of the toner image is decreased and a fillet may be formed at a leading edge or a trailing edge of a line. Thus, a high-quality toner image on the transfer material cannot be obtained.
It should be noted that the effect of the scattering of the toner may be decreased as the toner image is transferred onto the recording sheet via the intermediate transfer material. However, the scattering of the toner considerably deteriorates the toner image when the toner image is directly transferred onto the recording sheet without having the intermediate transfer material as is in image forming apparatus according to the present invention.